Amplifier fibers, and in particular fibers doped with rare earth elements such as erbium, are commonly used in long haul optical telecommunication systems in order to amplify transmitted optical signals. Such fibers are used in EDFAs (Erbium Doped Fiber Amplifiers) and have a central core composed of a silica matrix comprising Erbium doping elements optionally combined with complementary doping elements making it possible to improve the amplification.
In a manner known per se, optical amplification in an EDFA-type fiber operates by injecting into the fiber a pump signal which excites the (Er3+) ions of the doping element. When a light signal passes through this portion of optical fiber, it deexcitates the ions by laser effect by producing a photon identical in all respects with the incident photon. The light signal has therefore been doubled. In the particular case of EDFA amplifiers, only two wavelengths exist which can be used for optical pumping, namely 980 nm and 1480 nm. The 980 nm wavelength is usually used for low-noise equipment but the corresponding absorption window is relatively narrow, thus stabilized laser sources which are complex and expensive must therefore be used. The absorption window of the 1480 nm wavelength is wider but requires the use of very powerful lasers which are expensive.
It has therefore been sought to use other wavelengths to pump amplifier fibers, and in particular shorter wavelengths. A known solution involves using an energy transfer between co-doping elements having a significant overlap area in their absorption and emission spectra. For example a transfer between ytterbium and erbium (Yb/Er) elements or a transfer between semi-conductor elements and erbium elements.
The energy transfer between ytterbium and erbium elements for widening the absorption window of the pumping wavelength has in particular been described in the publications “Coherent effect of Er3+—Yb3+ co-doping on enhanced photoluminescence properties of Al2O3 powders by sol-gel method” by X. J. Wang et al., Optical Materials 26 (2004) 253-259 and “Optical gain of single mode short Er/Yb doped fiber” by Q. Wang et al., Opt. Express 12, 6192-6197 (2004). This solution is however limited to relatively high pumping wavelengths.
The energy transfer between semi-conductor elements and erbium for reduced pumping wavelengths has in particular been described in the publications “Visible Wavelength Emission in the Silica Glass Fiber Doped with Silicon Nano-particles” by Songbae Moon et al., ECOC 06. parer We3, P33, proceedings vol 3, p 187-188 and “Evidence of energy coupling between Si nanocrystals and Er3+ in ion-implanted silica thin films” by C. E. Chryssou et al., Applied Physics Letters, Vol. 75, No. 14, 4 Oct. 1999. This solution is however limited by the problem of maintaining the semi-conductor element in the reduced state.
For the two solutions of the prior art described above, the effectiveness of the energy transfer to the erbium atoms is limited to the closest neighbours, i.e. a distance of a few nanometers between the active species.
A need therefore exists for an amplifier optical fiber which allows the use of a reduced pumping wavelength, in particular in the visible range in order to be able to use low-cost sources.
Moreover, the energy transfer between the power of the pump signal and the emission by the Er3+ ions is limited to approximately 40%. It is therefore also sought to increase the signal amplification efficiency by increasing the intensity of the emissions by the rare earth ions, in particular by means of a longer range interaction between the ions involved in the transfer.
For this purpose, the invention proposes to exploit the phenomenon of electronic surface resonance, known as SPR or “Surface Plasmon Resonance”, of metallic nanostructures arranged in the core or in the vicinity of the core of the fiber. A light signal injected into the fiber will cause a vibration of the electron cloud surrounding the nanostructures; the free electrons surrounding the nanostructures can then resonate with the dielectric matrix of the core of the fiber. When the resonance wavelength corresponds to an excitation level of the rare earth element ensuring the amplification, an energy transfer between the pump signal and the amplified emission is ensured.
The phenomenon of electronic surface resonance SPR has already been observed. For example, the publication “Optical Properties of Gold Nanorings” by J. Aizpurua et al. Physical Review Letters, Vol 90, No. 5. 7 Feb. 2003, describes the optical response of gold nanoparticles in the form of rings arranged in a glass matrix.
Moreover, the publications “Surface plasmon polariton modified emission of erbium in a metallodielectric grating” by J. Kalkman et al., Applied Physics Letters, Vol 83, No. 1, 7 Jul. 2003, “Coupling of Er ions to surface plasmons on Ag” by J. Kalkman et al., Applied Physics Letters, Vol 86, 2005, 041113-1-3, and “Plasmon-enhanced erbium luminescence” by H. Mertens et al., Applied Physics Letters, Vol 89, 2006, 211107-1-3, describe an increase in the light intensity emitted by the erbium ions arranged in proximity to silver nanoparticles. It is thus possible to limit the thermal effects in a planar guide.
The publication “Assessment of spectroscopic properties of erbium ions in a soda-lime silicate glass after silver-sodium exchange” by A. Chiasera et al., Optical Materials 27 (2005) 1743-1747, also describes the effects of silver nanoparticles on erbium ions. This publication indicates that it was possible to use an excitation wavelength from 360 nm to 750 nm and that it was possible to observe an increase in the light intensity emitted by the erbium ions. However, this solution is not directly transposable to an application with optical fibers due to the incompatibility between the melting point of silver and the production temperature of the optical fibers.
This electronic surface resonance SPR phenomenon has thus never been used to excite the erbium ions in an amplifier fiber. The production constraints of the optical fibers impose choices on the nature, size and shape of the incorporated nanostructures.
The optical fibers comprising nanoparticles are moreover known from the prior art. For example, the documents EP-A-1 347 545 or WO-A-2007/020362 describe optical fibers comprising nanoparticles in the core of the fiber. The nanoparticles described in these documents include a rare earth doping element and at least one element improving the amplification of the signal, such as aluminum, lanthanum, antimony, bismuth or other.
These documents do not however describe metallic nanoparticles making it possible to create an electronic surface resonance SPR phenomenon in the core of the fiber.
Metallic nanoparticles have been used for optical sensors. For example, the documents U.S. Pat. No. 6,608,716 and U.S. Pat. No. 7,123,359 describe optical sensors comprising a doped medium and a plurality of aggregated nanoparticles for forming a fractal structure. The doped medium is not however an amplifier medium doped with a rare earth element, but a medium doped with atoms of metal, semi-metal, and/or semi-conductor.
The document U.S. Pat. No. 6,807,323 describes an optical sensor using the phenomenon of electronic surface resonance SPR between a thin film conductor and a dielectric thin film doped with rare earth elements or transition metals. This document does not however describe metallic nanoparticles arranged in a dielectric matrix doped with at least one rare earth element.
Thus, no document of the prior art describes an optical fiber comprising a core doped with at least one rare earth element and also comprising metallic nanostructures making it possible to create an electronic surface resonance SPR phenomenon in the core of the fiber in order to allow the use of a reduced pumping wavelength and/or in order to increase the energy transfer between the pump and the amplification.